Trans-nanosheet aggregation: unique electronic interactions between porphyrins mediated by 1-nm-thick titania nanosheets

In this study, the adsorption behavior and aggregation mechanism of two tetracationic porphyrins, tetra (N-methyl-4-pyridinium) porphyrin (p-TMPyP4+) and tetra(N-methyl-2-pyridinium) porphyrin (o-TMPyP4+), on titania nanosheets (TNS) were systematically investigated. Upon adsorption onto TNS, p-TMPyP4+ exhibited a stepwise red shift in the absorption maximum wavelength (λmax) with increasing adsorption density, indicating the formation of J-aggregates, whereas o-TMPyP4+ showed no such spectral change. A comparison with experiments using TNS monolayer films revealed that this aggregation behavior arises from electronic interactions mediated across the nanosheet interface rather than from direct molecular contact. This distinctive molecular assembly is defined as Trans-Nanosheet Aggregation, characterized by intermolecular electronic coupling through an ultrathin inorganic host approximately 1 nm thick. Based on Kasha’s dipole approximation model, the structural and energetic features of this aggregation were quantitatively analyzed, demonstrating that the TNS functions as a unique two-dimensional host capable of mediating ground-state electronic interactions between adsorbed molecules. These findings introduce a new concept of molecular aggregation across inorganic solids and provide a design strategy for next-generation photofunctional molecule/nanosheet hybrid materials.